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@ljharb We already had a reCAPTCHA on the page. It didn't help, and in fact even more spam accounts signed up after we added the CAPTCHA.

Slack is not designed for our use case at all (an open chat room where anyone can join), so it doesn't have proper tooling to help with this, and I doubt they'll ever add anything to help. For example, there's no way to turn off PMs, no way to have manual approval for signups, nor is there a way to ban people by IP directly in Slack itself.